Most modern computers include video monitor interfaces. A video memory subsystem is the portion of a computer that stores the image shown on the computer's video monitor. Video memory systems are designed to send the stored image data to the monitor at a rate synchronized with the raster scan rate of the monitor.
It is well known that color video memory subsystems are usually substantially more expensive than monochrome memory subsystems. There are two primary bases for the higher cost of color memory subsystems: (1) more memory is required to store color images than monochrome images, and (2) the monochrome memory support circuitry must be replaced with circuitry for supporting a color monitor. The second of these two items is the subject of the present invention.
Most desk top computer systems and workstations which can be used with monochrome monitors have video memory support circuitry that works only with monochrome monitors. To use the computer system with a color monitor, a new video memory subsystem must be added to the computer. In some cases, the monochrome memory subsystem is removed and replaced; in other computers the monochrome memory subsystem is not removable, and therefore the added color memory subsystem must deactivate or override the monochrome subsystem.
The differential in cost between color and monochrome video memory systems is therefore based, in part, on the need to add an additional subsystem on its own printed circuit board to the computer.
The present invention provides a video memory subsystem that works with both monochrome and color monitors, and thereby avoids a large portion of the cost associated with upgrading a monochrome system to a color system. This new video memory subsystem can be used with either a monochrome or color memory module the only difference being the amount of memory in the module, and a mode selection signal which denotes whether the memory module is a monochrome or color module. The video memory subsystem reads the mode selection signal and automatically selects a corresponding mode of operation.
It is therefore a primary object of the present invention to provide a video memory subsystem for both monochrome and color monitors, and to thereby avoid a large portion of the cost associated with upgrading a monochrome system to a color system.